Diborane (B.sub.2 H.sub.6) has been synthesized from BCl.sub.3 and H.sub.2 pyrolysis at high temperatures (550.degree. C-950.degree. C), by pyrolysis (650.degree. C) using various catalysts, and by electric discharge.
Diborane has also been prepared by reacting polymeric boron monoxide with gaseous hydrogen, in accordance with U.S. Pat. No. 3,021,197, at a temperature of about 850.degree. C to 1500.degree. C. The principal gaseous product of the reaction is diborane. Small proportions of other boron hydrides are obtained under some conditions. The diborane is separated by condensing at -196.degree. C after the other contaminants are separated in a cool zone prior to the condensation of diborane.
Diborane, also referred to as boroethane, has a molecular weight of 27.69. It is a colorless gas with a specific gravity when liquid of 0.45. The melting point of diborane is -169.degree. C; the boiling point is -92.5.degree. C.
Diborane is used as a reactant for making higher boron hydrides which are more stable and which are useful as fuels for engines and rockets. In addition to its usefulness as a starting material for making the hydrogen-boron compounds, diborane has usefulness as a reactant for making other boron derivatives since it is a highly reactive compound.
Because of diborane's highly reactive nature, however, higher boranes and other boron derivatives are formed from side reactions which take place, particularly when the synthesis takes place in a high heat bath which results in a high reaction temperature for the products as well as the reactants. These side reaction products are highly undesirable since they effect yield and complicate the separation procedures in obtaining diborane of high purity.
When diborane is synthesized from BCl.sub.3 and H.sub.2 a common long-lived intermediate, HBCl.sub.2, is formed. There may be other intermediates or unreacted products which require separation (e.g., H.sub.2, BCl.sub.3, B.sub.2 H.sub.n X.sub.m, etc. where n and m are intergers and X is chlorine); however, any contaminants complicate the separation and/or purification procedures for the highly reactive diborane.
Other investigators have attempted to make diborane by another method. For example, prior art investigators (Rockwood et al), using a pulsed CO.sub.2 multimode laser with a 1.5 w and 200 ns gain-switched pulse attempted to synthesize B.sub.2 H.sub.6, but without success.
It would be highly desirable and advantageous to synthesize diborane at a low reaction temperature since the production of undesirable intermediates would be lessened. Synthesis at low reaction temperatures would also permit better control of the desired product yield since many short-lived intermediates characteristic of high temperature reaction would complicate monitoring the progression of reaction.
An object of this invention is to provide a synthesis method which employs a laser induced chemical reaction for producing diborane at a low reaction temperature.
A further object of this invention is to provide a synthesis method which is accomplished at room temperature (e.g. 25.degree. C) by a laser induced chemical reaction to produce diborane while producing no higher boranes or polymers which are produced under certain conditions by side reaction when synthesis takes place at high reaction temperatures.